A coaxial electrical cable for high speed transmission of electric signals generally comprises a metallic inner conductor surrounded by a dielectric insulating material, which in turn is surrounded by an electrically-conductive outer material that provides a shield against passage of extraneous external electrical signals or noise which might interfere with signals carried by the inner conductor, or against passage of signals or noise generated by the inner conductor. Generally, a third layer of dielectric material surrounds the coaxial assembly which provides a sheath or jacket for protection against the use environment and to provide additional insulation. Coaxial electrical cables comprising the elements described above in single layers or in multiple coaxial layers are well known in the art.
As industry trends toward smaller and lighter weight coaxial cables formed of generally circular sequential layers of materials problems have arisen which are difficult to meet with current art without sacrificing desirable properties. One such problem is the maintenance of the position and alignment of the shielding materials during subsequent manufacturing steps. This can also be the case in the manufacture of coaxial cables having asymmetric shapes and other non-circular cross-section shapes.
Shielding materials may include metals or metallized plastic film in the form of wire, tape, or foil which are conventionally applied to surround a dielectric material layer by methods such as braiding, serving (helical wrapping), or folding (cigarette-style wrap). These materials are generally applied so that about 80% or more of the dielectric material surface is uniformly covered by the material in order to provide uniform shielding. If the shielding material is dislodged or its alignment distorted shielding effectiveness is reduced and the cable may be unusable.
In most cases in which wire, tape, or foil shielding materials are used the materials are sufficiently strong that they can be applied with enough back tension so that, once in place, they can resist the forces exerted on them by subsequent manufacturing steps and remain in place. However, when very fine wire or thin foils and tapes are used it is difficult to apply them to the surface of the dielectric material in a manner that resists displacement by subsequent manufacturing steps. In order to overcome these problems heavier gauge shielding materials may be required which runs counter to the desire for smaller, lighter, and more flexible cables.
The problem is more severe in cable constructions having served-wire or served-tape shielding, which are frequently desired for the flexibility which can be obtained in the cable. Served shielding is the most easily displaced type of shielding. Displacement of the shielding by subsequent manufacturing steps can also be a problem in the manufacture of coaxial cables that have non-circular cross-section shapes. Non-circular cross-section coaxial cable shapes such as are disclosed in U.S. Pat. No. 4,701,576 (to Wada, et al.) and U.S. Pat. No. 5,119,046 (to Koslowski, et al. ) may use pressure-extrusion methods to shape and mold the outer protective jackets. Such pressure-extrusion methods exert much higher forces on the coaxial cable materials already in place than are encountered in conventional extrusion of cable jackets and, consequently, are more likely to dislodge or displace shielding materials already in place.
It is an object of the invention to provide a coaxial cable in which the shielding material is held in place so as to not be disturbed or displaced by subsequent manufacturing operations or use.